Geoscience Reference
In-Depth Information
A distinctive characteristic of a gulf is that it usually has a re-
cessed shoreline that opens outward to a larger body of water.
Notice in Figure 19.1 how the shoreline of the Gulf of Mexico
curves northward and how the Gulf opens to the Caribbean Sea
east of Central America. On a still smaller level, a
bay
is a rela-
tively small indentation on the coast that is directly connected
to an ocean, sea, or gulf.
Although oceans, seas, and gulfs differ by size and their rela-
tionship to land, they are essentially interconnected and therefore
have the same basic chemical composition. This chemistry is a
result of complex interactions among several factors, including
the atmosphere, seawater, minerals, sediments, and the myriad
of organisms living in the water. Given that water is an excel-
lent solvent, the world's oceans and seas contain a wide variety
of dissolved solids, such as chlorine, sodium, and magnesium
ions, among others. The term
salinity
refers to the concentra-
tion of dissolved solids in seawater and is most commonly ex-
pressed in parts per thousand. Overall, global salinity varies
between 34‰ and 37‰. Water that exceeds a salinity of 35‰
is considered
brine
, whereas water with salinity less than 35‰
is called
brackish
.
Figure 19.2 A typical coastline in northern California.
Coastlines such as this one at Big Sur are where the hydro-
sphere, lithosphere, and atmosphere meet in a very big way.
spatial scales—some movements occur very slowly over cen-
turies, whereas others are related to daily oscillations and flow
patterns. This section discusses the primary ways in which wa-
ter moves along a coastline.
The Nature of Coastlines:
Intersection of Earth's Spheres
Fluctuations in Water Level
Over long periods of time,
water levels in oceans and large lakes change for a variety of
reasons. For instance, changes can occur when tectonic forces
cause a landmass to be uplifted or sink. Water levels also fluc-
tuate when the amount of water in the ocean or lake varies as
a result of adjustments in the hydrologic cycle. This form of
water-level fluctuation is called
eustatic change
.
The best example of eustatic change in recent Earth his-
tory is the great change in sea level that occurred at the end of
the Pleistocene Epoch, when the massive continental glaciers
melted and returned water to the oceans. Figure 19.3 shows that
sea level changed dramatically between the Illinoisan glaciation
(called the
Riss glaciation
in Europe) around 130,000 years ago
and the Holocene Epoch (the past 10,000 years). Major periods
of low ocean water level occurred during both the Illinoisan/
Riss glaciation and Last Glacial Maximum (the Wisconsin
glaciation discussed in Chapter 17), when enormous volumes
of water were stored in the immense ice sheets that covered
much of North America and Europe. When the ice subsequently
melted, the sea level rose rapidly because water poured back
into the ocean.
The primary distinguishing characteristic of coastlines is that
they are places where landmasses and large water bodies inter-
sect (Figure 19.2), often for hundreds or even thousands of kilo-
meters. Although coastlines are usually associated with oceans
and seas, they also occur around large lakes such as the Great
Lakes of North America. Coastlines are especially noteworthy
because they represent the intersection of three major Earth
spheres, namely the hydrosphere, lithosphere, and atmosphere.
Coastlines come in all shapes and sizes. Some coasts have
wide, sandy beaches, whereas others feature rocky bluffs and
cliffs. Along some coastlines, mountains extend straight down
to the water. Because of the wide range of environments as-
sociated with coastlines, some are passive and change slowly,
whereas others erode vigorously and are in a near-constant state
of adjustment.
Processes That Shape the Coastline
A variety of physical processes influence the shape of a coast-
line, with most directly related to how water moves along the
shore. This movement occurs at several different temporal and
Water that has salinity less than 35‰.
Bay
An indentation in the shoreline that is generally associ-
ated with an ocean, sea, or gulf.
Brackish
Eustatic change
Fluctuations in sea level associated with
adjustments in the hydrologic cycle.
Salinity
Concentration of dissolved solids in water that is
measured in parts per thousand (‰).
Water that has salinity greater than 35‰.
Brine
